This invention relates to apparatus for making cooled pourable concrete. It is desirable that the temperature of fresh concrete be low. In particular, the characteristics of concrete made in summer with daytime temperatures above 77.degree. F. (25.degree. C.) are improved by cooling. Specifically, hot weather concrete has the following drawbacks:
(1) reduction in slump (slump is the height lost in a mound of concrete poured into a truncated conical form when the form is removed according to JIS A 1101 specifications; the softer the concrete, the greater the slump.) PA1 (2) cracking, and PA1 (3) reduction in strength.
Water evaporation is responsible for slump reduction. For example, it is reported that 18 cm. slump concrete has a 6 cm. reduction in slump when agitated in a truck agitator for about one hour with an 86.degree. F. (30.degree. C.) temperature when mixed. When slump is reduced, pouring becomes difficult. It becomes necessary to add cement paste and remix the concrete. Further, fresh concrete has the characteristic that even when the amount of added water is adjusted to be the same, the slump is still reduced as temperature is increased.
Cracking during hardening is caused by heat generation within the interior of the concrete. The hydration reaction that occurs when concrete hardens is an exothermic reaction. When heat is generated in the interior, a temperature differential is created between the interior and outer surfaces of the concrete. Expansion of the heated interior and contraction of the cooled outer surfaces generates cracks.
The cracking of concrete has detrimental effects on every application. Particularly, in the case of structures such as dams, bridge supports under the ocean, and walls of nuclear reactors, cracks can be a fatal flaw.
Further, hot weather concrete has reduced strength when hardened.
Drawbacks such as these can be eliminated by cooling the fresh concrete. Apparatus which cool the water added to the concrete have been developed for cooling fresh concrete. However, the temperature of the concrete cannot be cooled significantly by cooling the added water. This is because the amount of water added to the concrete is only 4% to 6% of the entire mixture.
Apparatus which cool the cement that is added to fresh concrete have also been developed. In these apparatus, the cement is forcibly cooled by blowing liquefied nitrogen gas into it. These apparatus have the feature that the cement can be cooled without adding water, but they have the drawback of extremely high running costs. The reason for this is the large consumption of expensive liquid nitrogen. Therefore, these apparatus can only be used for special purpose concrete.
Of all the materials added to concrete, aggregate or gravel is the largest component by weight. Therefore, the most effective way to reduce the temperature of fresh concrete is to cool the aggregate. Apparatus for cooling aggregate have been developed to reduce this idea to practice.
An apparatus which utilizes the heat of vaporization of aggregate surface water has been developed as (Japanese Patent disclosure 188317/1982). In this apparatus, aggregate is loaded into an airtight tank, air is evacuated from the tank forcibly vaporizing the water, and the aggregate is cooled by the vaporization of the water. This apparatus has the drawback of high equipment cost incurred by the requirement for a large pressure tank and a high volume vacuum pump. It also has the drawback that the aggregate cannot be cooled in a continuous fashion.
In addition, cooling apparatus which discharge liquefied nitrogen gas onto the aggregate have been developed (Japanese Patent disclosures 156045/1988, 26407/1989, and 26408/1989). Apparatus such as these have the feature that cooling can be achieved without adding water to the aggregate. Further, they have the feature that the aggregate can be cooled to a low temperature. However, like apparatus which cool the cement with liquefied nitrogen gas, these apparatus also have have the disadvantage of high running costs, and therefore can be used only for special applications.